The present invention relates to systems for controlling the quantity of electron beam current in a camera tube in accordance with a video signal derived from the camera tube.
Television cameras currently in use generally include one or more image pickup tubes to convert an optical image into an electrical video signal. One type of image pickup tube conventionally used includes a photosensitive surface, known as a target, upon which the image is focused. An electron beam scans the target and deposits electrons thereon to charge the surface to a uniform electrostatic potential. Between successive scans by the electron beam, each elemental portion of the target surface will discharge by an amount determined by the intensity of light falling on that portion of the target. Because of this, the light image focused on the target will cause a corresponding positive potential electrostatic image to be formed on the target. As the electron beam scans over the target, current will flow from the electron beam through the target. The magnitude of this current will be determined by the amount of current required to charge that portion of the target being scanned by the electron beam back to the uniform potential. The current flowing from the target electrode, usually referred to as the signal current, thus varies in accordance with the pattern of light imaged on the pickup tube, and represents the video signal which is processed and transmitted to the remote television receiver.
In conventional practice, the magnitude of the current flow in the electron beam scanning the target is fixed at a level which is somewhat greater than the level necessary to recharge the target when it is exposed to a normal intensity light image. When the electron beam current is set at this level, however, the imaging tube is incapable of completely recharging those portions of the target which have been exposed or higher intensity portions of the image. This condition, known as beam shortage or beam starvation, gives rise to such undesirable phenomenon as comet-tailing and blooming in the subsequently reconstructed image. Although this effect could be reduced by increasing the intensity of the electron beam, to do so would result in defocusing of the beam. This would also shorten the useful life of the pickup tube, which is inversely related to the average beam intensity.
More recently, systems have been devised wherein the video signal is fed back to the system which controls the beam current so that the beam current is dynamically varied in accordance with the video signal derived from the target. In this manner, the intensity of the electron beam is automatically set at a higher level when the signal current indicates that a highlight is present at that point on the target. A system of this general type is disclosed in the patent to Sato et al., U.S. Pat. No. 3,999,011.